Humans aren't the only animals that change their environments

Posted: Sunday, March 04, 2007

Organisms that physically modify elements of ecosystems are often called ecosystem engineers. Beavers, the subject of a previous column in this space, are a prime example. There are many others, however, with effects ranging from very widespread to very local.

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At the small end of the scale are such things as woodpeckers, which make holes in trees, creating nest sites for many other organisms and entry points for bacteria and fungi that may eventually rot the tree.

Fiddler crabs in salt marshes dig burrows, increasing decomposition rates, drainage, and plant production.

Bears dig up the roots of skunk cabbage, leaving a pit that fills with water and alters local drainage patterns.

Game trails on steep slopes can create erosional pathways, and seabird nesting burrows are reported to increase soil erosion dramatically in some places.

Even tiny organisms can be engineers. The algae, fungi, and bacteria that form crusts on the surface of the soil in otherwise barren locations secrete complex carbohydrates that bind soil particles together.

These crusts control erosion, runoff, and sites for seed germination. They are visible in many places near the Mendenhall Glacier.

On some beaches, tiny one-celled algae called diatoms produce complex carbohydrates that link sand grains together, which helps to stabilize the beaches. Organisms that eat diatoms or algae obviously have the opposite effect of destabilizing the soils or sands.

Geese grub up roots and basal stems of grasses and sedges in salt marshes. Studies of the Lesser Snow Goose in Canada have shown that at low densities, grubbing by snow geese changes the vegetation composition by favoring certain kinds of plants; deposition of digested plant material in goose feces increases nitrogen cycling and encourages reestablishment of plants.

However, continual high densities of foraging geese cause elimination of vegetation over wide areas of marsh, which leads to soil erosion and creation of barren areas where the original vegetation is unable to reestablish.

Burrowing mammals are earth-moving engineers. They can move huge quantities of soil. For example, gophers and ground squirrels have moved up to 40 tons per acre of soil in some locations.

These activities change vegetation succession, increase habitat heterogeneity, and increase the variety of plants that occupy the areas.

Salmon are engineers too. Females dig nests that can be many inches deep, moving fine sediments downstream. Sorting the sediments is thought to decrease the chance that floods will scour the streambed down to bedrock.

Spawning king salmon in some rivers have built up huge dunes of gravel by their digging activities. Such dunes can be several feet high and many feet long, so they alter streamflow patterns.

Earthworms are prodigious movers of soil. Charles Darwin wrote a whole book about earthworms and their engineering effects. Several ounces of soil may be deposited at the mouth of a single worm burrow.

The cumulative effect of many worms can add many inches of soil on the surface, with the result that stones lying on the surface appear to sink and are gradually buried.

In agricultural systems, their engineering effects are commonly considered to be largely beneficial (to humans, that is). Worms help decompose vegetation and incorporate that organic material in the soil; their tunnels aerate the soil.

When glaciers covered much of North America, earthworms were wiped out by grinding or freezing, not only under the glacier itself but also in the adjacent cold zones.

So then there were no native earthworms in North America except outside the regions affected by glaciers. Native worms have gradually moved into deglaciated regions, but they spread slowly (on the order of a few yards per year), and much of northern North America still has no native earthworms. No native earthworms are known to occur in Alaska soils.

Earrthworms are often introduced to areas by gardeners and farmers, to make compost and improve nutrient cycling, or by horticulturalists in potted plants; in some regions they are introduced by fishermen who use them as bait.

At least 45 species of exotic earthworms have been introduced to North America, and different species have different tolerances of cold weather, different rates of spread, and different effects on ecosystems.

In general, earthworm invasions have had marked effects on soil processes, especially in areas that previously had no earthworms. For example, studies in cool temperate forests have demonstrated that introduced worms resulted in lower soil nitrogen availability, which was reflected in decreased nitrogen in the leaves of several plants; lower nitrogen in leaves is likely to influence the foraging of leaf-eating animals.

In some areas, leaf litter or even all of the soil on the forest floor was eliminated entirely, decreasing the availability of habitats for forest-floor invertebrates and sites for seed germination.

Exotic worms also have changed the abundance and species composition of mycorrhizal fungi, so important for plant growth. They are known to carry pathogens that affect both plants and humans - as well as helping the dispersal of disease-control agents.

Humans are the quintessential ecosystem engineers. They physically modify almost every landscape they occupy.

Humans drain marshes and entire aquifers, dewater some streams and flood others, cut or burn broad swathes of trees, plow up the sod, quarry rock and gravel deposits, cut away whole mountains in search of minerals, pave the ground with impermeable surfaces, dig harbors and build breakwaters, send untold millions of tons of sediment down rivers into bays and seas, fill lagoons and lakes and swamps, and so on.

Human engineering typically has negative effects on previous occupants of an area.

• Mary F. Willson is a retired professor of ecology and a Trail Mix board member.



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